It has been known for many years that feed solutions comprising various hydrocarbon compounds and a solvent may be separated by contacting such a feed solution with one side of a membrane and recovering one or more hydrocarbon compounds and, in some cases solvent, from the other permeate side of the membrane.
It is also known that a solvent may be applied in the feed solution to increase the rate of permeation of organic compounds through particular elastomeric membranes. The applied solvent causes the membrane to swell to such an extent that (polymeric) organic compounds having relatively high molecular weights (e.g. above 1000) will permeate together with the solvent through relatively thick barrier membranes on the order to millimeters. Such a process is disclosed in U.S. Pat. No. 3,440,264 where organo-polysiloxanes such as 3,3,3-trifluoropropyl methyl siloxane are used for separation membranes. The applied solvent is not separated in such a process; hydrocarbons remain present in the solvent either at the permeate side of the applied membrane and at the feed side. The patent does not disclose the use of a combination of aromatic and polar aliphatic solvents to render polysiloxane membranes substantially impermeable to hydrocarbon oils.
Surprisingly, it has now been found that a particular combination of solvents, i.e. an aromatic solvent together with a polar aliphatic solvent, can be separated with excellent selectivity from dissolved hydrocarbons such as oil fractions. The separation is effected by applying a dense substantially hole free membrane which is substantially impermeable to the hydrocarbons. The membrane comprises a layer of a halogen-substituted silicon compound as defined below.
It has also been found that the rate of permeation of the solvents through the membrane, the flux, can be susprisingly high in the process according to the invention.